A magnetic resonance imaging device produces a measurement of a sample which is based upon its molecular structure. The sample is subjected to a polarizing magnetic field which has the effect of aligning the spins of all the atomic nuclei of the sample. Radio waves at a frequency close to the Lamor frequency of the nuclei are then used to excite the nuclei such that their magnetic alignment is reversed. Once the excitation is removed the nuclei return to their original state by emitting characteristic radio signals. It is these radio signals that can be used to image the sample.
The exact Lamor frequency is dependent upon the precise magnetic field. By creating a magnetic field gradient within the sample cavity the source of these signals can be located such that an overall image of the sample can be constructed.
The efficiency of this process depends upon the consistency of the magnetic field strength within the sample cavity. This field is typically controlled to within 5 parts per million. The extent of the uniformity of the magnetic field determines the accuracy with which the Lamor frequency can be measured. This allows for the resolution of smaller chemical shifts.
Uniformity of magnetic fields to less than 5 ppm is very difficult to achieve over a large volume. In order to overcome fluctuations in the permanent magnetic field produced by a surrounding magnetic circuit, a variety of shimming methods may be used, such as the inclusion of ferromagnetic material of specific shapes at specific locations for example as described in patent no. GB 2,378,763 or the use of oddly shaped shimming coils whose magnetic fields can be adjusted by controlling the current passing through them.
In order to superimpose a magnetic field gradient onto the uniform magnetic field in the sample cavity, additional gradient coils are used. The higher the magnetic gradient that can be produced by the gradient coils, the higher the resolution of the image acquired, however in order for the gradient to cover the whole sample cavity requires a large coil.
A uniform magnetic field can be achieved by the methods described above, however maintaining its uniformity in a fluctuating environment requires a dynamic shimming mechanism. For example, in an industrial setting where, say, a high temperature liquid sample is being measured and the magnetic permittivity of sample may be temperature dependent, the magnetic field characteristics around the sample will change over time. In this environment it is necessary to use a dynamic shimming mechanism which is able to respond quickly to changes in the sample environment.
There is thus a long felt need in the art for a method of correcting the shimming elements of a magnetic resonance device in real time so as to counter deviations in the magnetic field around the sample.